1. Field of the Invention
The present invention is related to a current mode buck converter, and more particularly, to a current mode buck converter adaptively adjusting a trigger condition for a pulse frequency modulation mode according to an average of an inductor current or AC components of the inductor current and a slope compensation current, to prevent variations of an input voltage, an output voltage and the inductor current.
2. Description of the Prior Art
An electronic device generally includes various components requiring different operating voltages. Therefore, a DC-DC voltage converter is essential for the electronic device to adjust (step up or step down) and stabilize voltage levels. Based upon different power requirements, various types of DC-DC voltage converter, originating from a buck (step down) converter and a boost (step up) converter, are developed. Accordingly, the buck converter can decrease an input DC voltage to a default voltage level, and the boost converter can increase an input DC voltage. With advances in circuit technology, both the buck and boost converters are varied and modified to conform to different system architectures and requirements.
For example, please refer to FIG. 1, which is a schematic diagram of a buck converter 10 of the prior art. The buck converter 10 includes an input end 100, a switch module 110, an output module 120, an output end 130, a feedback module 140, an error amplifier 142, a voltage reduction circuit 144, a pulse width modulation (PWM) compensation circuit 146, a current sensor 150, a current sense circuit 152, a slope compensation circuit 160, a first comparator 170, a second comparator 180, a third comparator 190, an oscillator 192 and a modulation control circuit 194. The input end 100 is utilized for receiving an input voltage VIN. The switch module 110 is utilized for determining whether the input end 100 or a ground GND is electrically connected to the output module 120 according to a switch signal SW. The output module 120 is utilized for generating an output voltage VOUT based on frequency responses of an output inductor 122, an output resistor 124 and an output capacitor 126 and a conducting state of the switch module 110. The feedback module 140 is utilized for generating a divided voltage of the output voltage VOUT as a feedback signal VFB. The error amplifier 142 is utilized for amplifying a voltage difference between the feedback signal VFB and a first reference voltage VREF1 to generate a differential voltage ΔV. The voltage reduction circuit 144 is utilized for generating a divided voltage VREF1′ slightly lower than the first reference voltage VREF1. The second comparator 180 is utilized for comparing the divided voltage VREF1′ and the feedback signal VFB to generate a PWM trigger signal TR_PWM. With respect to feedback schemes other than the feedback signal VFB, the current sensor 150 detects an inductor current IL of the output inductor 122 to generate a sensing current ISEN. The current sense circuit 152 amplifies the sensing current ISEN to generate a mirror inductor current IL_C. The slope compensation circuit 160 is utilized for generating a slope compensation current ISC. A sum of the mirror inductor current IL C and the slope compensation current ISC is converted into a sensing voltage VC by a resistor R. The PWM compensation circuit 146 is utilized for compensating a frequency response of the buck converter 10 according to the differential voltage ΔV to generate a compensation result EAO. The first comparator 170 is utilized for comparing the sensing voltage VC and the compensation result EAO to generate a PWM signal VPWM. The third comparator 190 is utilized for comparing the compensation result EAO and a fixed threshold voltage VTH to generate a pulse frequency modulation (PFM) trigger signal TR_PFM. The oscillator 192 is utilized for generating an oscillating signal VOSC. Finally, the modulation control circuit 194 determines which operation mode the buck converter 10 operates in based on the PFM trigger signal TR_PFM, the PWM trigger signal TR_PWM, the PWM signal VPWM and the oscillating signal VOSC, and generates the corresponding switch signal SW sent to the switch module 110.
In short, the buck converter 10 determines whether to operate in a PWM mode or a PFM mode based on the inductor current IL. When the inductor current IL is relatively low, the buck converter 10 switches from the PWM mode to the PFM mode to reduce a switching loss of the buck converter 10 by minimizing switching operations of the switch module 110. The buck converter 10 generates the PWM trigger signal TR_PWM and the PFM trigger signal TR_PFM according to the sensing current ISEN and the feedback signal VFB, and accordingly determines whether to operate in the PWM mode or the PFM mode.
A period of the PWM signal VPWM is formed based on a period of intersection points of the compensation results EAO and peaks of a sum of the mirror inductor current IL_C and the slope compensation current ISC, as illustrated in FIG. 2. However, the sum of the mirror inductor current IL_C and the slope compensation current ISC varies with the input voltage VIN, the output voltage VOUT and inductance of the output inductor 122. In such a situation, the compensation result EAO has to be adjusted accordingly. That is, for the third comparator 190, a trigger condition for the PFM mode varies with the input voltage VIN, the output voltage VOUT and the inductance of the output inductor 122. For example, under a condition that the input voltage VIN and the output voltage VOUT are invariant, the larger the output inductor 122, the higher a current threshold Ith1 specifying a decision boundary from the PWM mode to the PFM mode, as illustrated in FIG. 3. In the worst case, the current threshold Ith1 is even greater than a current threshold Ith2 specifying a decision boundary from the PFM mode to the PWM mode, causing the buck converter 10 to oscillate between the PWM mode and the PFM mode and malfunction. To prevent the mode oscillation, one approach is to decrease the threshold voltage VTH. However, the threshold voltage adjustment probably results in a very small current threshold Ith1, implying that the PFM mode is inaccessible.
Therefore, fixing the decision boundaries between the PWM mode and the PFM mode has been a major focus of the industry.